Power efficiency of a power amplifier is a crucial issue in wireless communication systems. A stand-alone class-A PA suffers the problem of low power efficiency. On the other hand, a stand-alone power efficient PA, like class-AB or class-B amplifier, is usually highly nonlinear. When a non-constant-envelope modulated signal goes through a nonlinear PA, inter-modulation distortion (IMD) will emerges. This not only distorts the modulated signal but also causes the power spectrum of the modulated signal to overflow to the adjacent channels. As a result, both self-interference and mutual-interference among neighboring channels seriously degrade the communication quality. In order to maintain power efficiency and suppress IMD, it is a common practice to adopt a nonlinear PA with high power efficiency.
There exist a few schemes for PA linearization, such as the feed-forward scheme, the feedback scheme, and the predistortion scheme. Each is with either analog approaches or digital approaches. Generally speaking, the feed-forward schemes are costly and the feedback schemes are limited to only narrow band applications. All the analog approaches are inflexible. Therefore, in terms of cost effectiveness, the digital predistortion schemes are superior to the others.
Shown in FIG. 1 is a block diagram illustrating the linearization of a digital predistorter (PD). The digital PD 101 predistorts a modulated input signal vm to invert the nonlinear distortion introduced by a PA 107. In particular, a digital adaptive PD (DAPD) employing a gain-based look up table 101a is very attractive for its flexibility in algorithm adaptation and its high accuracy in nonlinear compensation. As shown in FIG. 1, the complex baseband modulated input signal vm carrying the payload data is fed to the cascade of the PD 101 and a radio frequency (RF) link. The PD 101 distorts the modulated input signal vm to produce a predistorted signal vd. The RF link takes over the predistorted signal vd, to generate the transmission signal va, through a digital-to-analog (D/A) converter 103 for transformation, a quadature modulator 105 for frequency up-conversion, and the PA 107 for power amplification.
Because the characteristics of a PA may vary with temperature and may be affected by aging, an adaptive algorithm is required in a DAPD-LUT scheme to update the LUT entry values. In addition, the linearization accuracy of a DAPD-LUT scheme in terms of IMD will improve 6 dB if one doubles the number of LUT entries. However, the more LUT entries one adopts, the lower LUT convergence speed it will suffer.
Several gain-based LUT techniques are either analyzed or implemented. FIG. 2 is a block diagram illustrating a conventional gain-based DAPD-LUT technique that the indexing of the N-size LUT entries is uniformly spaced, wherein the normalized unsaturated input amplitude range of a PA is [0, 1] and an LUT entry's spacing di equals to 1/N. However, in the uplink or downlink of a wireless network, most transmitted signals do not occupy the input amplitude range of the entire PA. Some LUT entries will never be selected. Therefore, a non-uniform LUT spacing technique is highly desired to avoid wasting LUT entries.
FIG. 3 is a block diagram illustrating a conventional gain-based DAPD-LUT technique with an optimum non-uniform LUT spacing, wherein the LUT is indexed by the input amplitude rm of input modulated signal via a mapper S(rm) to implement a non-uniform LUT spacing di, which is referred to as the conditionally-optimum spacing technique. The technique assumes knowledge of the conditions on the input signal backoff (IBO), the PA characteristics, and the probability density function (PDF) of the modulated input signal. When any of the assumed knowledge varies with time, the optimum LUT spacing needs to be recalculated. Unfortunately, the computational complexity of recalculating the LUT spacing in such a conditionally optimum technique is pretty high.
Since the conditionally optimum technique is optimum only under a specific set of conditions, any condition mismatch could cause significant performance degradation. However, some of conditions are difficult to accurately obtain, e.g. the PA characteristics, and some of conditions can be fast time-varying, e.g. the IBO. In addition, the computational complexity of the conditionally optimum technique thwarts any attempt to online optimize the LUT spacing for a different set of conditions. Therefore, an unconditionally optimized technique is practically useful.
FIG. 4 is another conventional gain-based DAPD-LUT technique with a non-uniform LUT spacing, which is referred to as the piecewise-uniform spacing technique. In the piecewise uniform spacing technique, the whole unsaturated PA input amplitude range is first artificially divided into several segments, such as 4 segments S1-S4, according to the nonlinearity of the PA characteristic curve. Each of those nonlinear segments will be assigned more LUT entries than each of those linear segments to combat the PA nonlinear distortion. Although it is still uniform spacing within each segment, this technique as a whole enjoys the advantage of non-uniform LUT spacing. The piecewise-uniform spacing technique also requires prior knowledge of the PA characteristic so as to divide the PA input amplitude range into segments of different linearities. The piecewise-uniform spacing technique focuses on the subject of PA characteristics and ignores how input signal statistics may influence the IMD performance of a PA linearization technique.
Because of the aforementioned problems, it is imperative to provide a technique to dynamically calculate an unconditionally-optimum LUT spacing which minimizes the overall average IMD power.